Quantitative serial assessment of patellofemoral joint osteoarthritis from lateral knee radiographs

This study describes a new method for patellofemoral joint space measurement on lateral knee radiographs of patients from a case register of knee osteoarthritis (OA). The reproducibility of the technique was tested on 31 lateral radiographs and the validity, accuracy and sensitivity to change were examined in 82 patients (52 female, 30 male, mean age 66.5 years) with anteroposterior and lateral knee radiographs at two time points 3 years apart. The measurements were compared with visual assessments of joint space narrowing, subchondral sclerosis and osteophyte grade previously recorded by an independent observer. The degree of knee flexion was also recorded. The 'narrowest' joint space measurement was found to be a simple reproducible technique (coefficient of variation 8.5%). A significant correlation was found between the measured joint space and knee flexion. The reduction in measured joint space over 3 years was not significant (P = 0.067) but the group contained a large number of disease-free patellofemoral joints. The joint spaces were, however, significantly smaller in the presence of patellofemoral OA with or without tibiofemoral OA suggesting the technique to be valid. The measurements were also significantly smaller in the presence of a visual assessment of joint space narrowing and subchondral sclerosis. The mean measured joint space increased in the presence of severe osteophyte formation. This suggests that patellofemoral joint space measurement from lateral radiographs provides a sensitive and quantitative assessment of progression in OA at this articulation.